Traditionally, signal exchanges within telecommunications networks and data communications networks have been accomplished by transmitting electrical signals via electrically conductive lines. However, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. Data communications networks often utilize packet switching techniques in which information is separated into fixed-length packets that are transmitted and then reassembled at the destination site. An advantage of packet switching is that it efficiently uses the transmission resources of a system. A disadvantage with respect to optical packet transmissions is that the switching equipment is not as well developed as comparable electrical switching equipment. In particular, the absence of easy-to-use optical memory is a major drawback. Optical cross-connect circuit switches are used by telecommunications companies to route high bit-rate signals between optical fibers. They are particularly useful in systems employing wavelength division multiplexing (WDM), in which information capacity is increased by using parallel channels at different wavelengths. Switches of this type are commercially available, but suffer from either large size, poor performance, high price, or a combination of these factors.
U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from any one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. A functionally related matrix of switching elements is described in U.S. Pat. No. 4,988,157 to Jackel et al. A switching element 10 is shown in FIG. 1, while a 4.times.4 matrix 32 of switching elements is shown in FIG. 2. The optical switch of FIG. 1 is formed on a substrate. The substrate may be a silicon substrate, but other materials may be used. The optical switch 10 includes planar waveguides defined by a lower cladding layer 14, a core 16 and an upper cladding layer 18. The core is primarily silicon dioxide, but with other materials that affect the index of refraction of the core. The cladding layers should be formed of a material having a refractive index that is substantially different from the refractive index of the core material, so that optical signals are guided along the core material.
The core material 16 is patterned to define an input waveguide 20 and an output waveguide 26 of a first waveguide path and to define an input waveguide 24 and an output waveguide 22 of a second waveguide path. A trench 28 is etched through the core material to the silicon substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the trench is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide 20 to the output waveguide 22, unless an index-matching material is located within the gap between the aligned segments 20 and 26. Ideally, the trench 28 is positioned with respect to the four waveguides such that one sidewall of the trench passes directly through the intersection of the axes of the waveguides. In the 4.times.4 matrix 32 of FIG. 2, any one of four input waveguides 34, 36, 38 and 40 may be optically coupled to any one of four output waveguides 42, 44, 46 and 48. The switching arrangement is referred to as "non-blocking", since any free input fiber can be connected to any free output fiber regardless of what connections have already been made through the switching arrangement. Each of the sixteen optical switches has a trench that causes TIR in the absence of an index-matching liquid, but collinear segments of a particular waveguide path are optically coupled when the gaps between the collinear segments are filled with an index-matching fluid. Trenches in which the waveguide gaps are filled with fluid are represented by fine lines that extend at an angle through the intersections of optical waveguides in the array. On the other hand, trenches in which there is an absence of index-matching fluid at the gaps are represented by broad lines through a point of intersection.
The input waveguide 20 of FIGS. 1 and 2 is in optical communication with the output waveguide 22, as a result of reflection at the empty trench 28. Since all other cross points for allowing the input waveguide 34 to communicate with the output waveguide 44 are in a transmissive state, a signal that is generated at input waveguide 34 will be received at output waveguide 44. In like manner, input waveguide 36 is optically coupled to the first output waveguide 42, the third input waveguide 38 is optically coupled to the fourth output waveguide 48, and the fourth input waveguide 40 is coupled to the third output waveguide 46.
There are a number of available techniques for changing an optical switch of the type shown in FIG. 1 from a transmissive state to a reflective state. In the above-identified patent to Jackel et al., water or a refractive index-matching liquid resides within the gap between waveguides until an electrochemically generated bubble is formed. A pair of electrodes are positioned to electrolytically convert the liquid to gaseous bubbles. A bubble at the gap between collinear waveguides creates an index mismatch and causes light to be reflected at the sidewall of a trench. The bubble can be destroyed by a second pulse of appropriate polarity, thereby removing the is bubble and returning the switch to the transmissive state.
Japanese application No. 6-229802 of Sato et al. (Kokai No. 8-94866) describes the use of heaters to supply and remove index-matching liquid to and from a gap that is intersected by two waveguides. Flow of liquid is controlled by selectively activating heater elements.
Sources of signal loss at a switching matrix include coupling losses at the interfaces between optical fibers and waveguides, transmission losses along the waveguides, and transmission losses as a result of crossing a fluid-filled trench from one waveguide to a collinear waveguide. A conventional optical fiber has a diameter of approximately 8 .mu.m. In order to control coupling losses, the cores of waveguides may be fabricated to substantially match the diameter of the optical fiber. The above-identified reference to Sato et al. describes fabricating a core layer to have a square cross sectional configuration, with both the thickness and width of the waveguide being approximately 8 .mu.m. The text of the Jackel et al. patent describes the core layer as having a thickness of approximately 7 .mu.m, which approximately matches the diameter of a single mode fiber. Abrupt dimensional changes would lead to both signal loss and signal reflection.
The selection of waveguide dimensions to minimize coupling losses at the interfaces between optical fibers and waveguides is not necessarily a desirable selection with respect to optimizing other aspects of the switching function. Consequently, tradeoffs need to be made. Depending upon the width of trenches that intersect the waveguides, wider waveguides may reduce the transmission losses across the fluid-filled gaps. For example, in applications in which higher losses can be tolerated (e.g., data communications), it may be preferable to interconnect standard 8 .mu.m core optical fibers to wider and thicker waveguides, thereby accepting coupling losses in exchange for increased performance provided by having larger cross sectional dimensions at the interface with the trench.
What is needed is an optical switching element that does not sacrifice performance in the selections of waveguide dimensions, gap dimensions and fiber dimensions. That is, what is needed is an optical switching element that provides both low fiber-to-waveguide coupling losses and low transmission losses through a fluid-filled gap between a pair of waveguides.